A process for treating a semiconductor wafer such as a silicon wafer includes controlling an in-plane temperature distribution of the silicon wafer as desired while controlling the temperature of the silicon wafer to a temperature setpoint.
For this purpose, there has been known a method of controlling the temperature of the semiconductor wafer simultaneously using a plurality of temperature adjusters through independent control loops respectively provided to the temperature adjusters.
Regarding the above temperature control for a semiconductor wafer, it is necessary that control variables should have a certain error from a reference control variable until the temperature reaches a setpoint and that the temperature should be maintained at the setpoint irrespective of any disturbance. In connection with the above, a master-slave control method is typically known (see, for instance, Patent Literature 1: JP-A-7-200076).
In the master-slave control method, one of a plurality of control loops is controlled as a master, and an error between a control variable (a setpoint) of the master loop and a control variable of the rest (a slave loop(s)) of the control loops is calculated and controlled so that the slave loop(s) follows the behavior of the master loop.
Usually, the control loop having the slowest response speed is defined as the master loop and the rest of the control loops is defined as the slave loop following the master loop.
When the master-slave control method is applied to a plate-shaped temperature adjustment device for a semiconductor which includes a plurality of heating and cooling zones, temperatures of all the zones are made uniform, or alternatively, the plate is made to have temperature gradient with different temperature setpoints for the zones, depending on usage. For instance, when the plate is placed in a chamber, the semiconductor wafer is liable to be affected by heat of walls of the chamber, so that a periphery of the plate may be easily heated than the center thereof.
In such a case, a temperature setpoint of a central zone of the plate needs to be set high while a temperature setpoint of a peripheral zone of the plate needs to be set low. For this setting, an offset temperature, which is suitable for a temperature adjuster in the slave loop relative to a temperature setpoint of a temperature adjuster in the master loop, is set for adjustment.
However, although described in more detail later, in the master-slave control method, heating and cooling in the plate with a temperature gradient cause problems below.
Specifically, when a control loop having the highest temperature setpoint is set as the master loop in the heating, the slave loop is set to have a temperature setpoint that includes a certain offset against the temperature setpoint of the master loop. Accordingly, when the master loop starts to be executed for heating, the slave loop temporarily starts being executed for cooling against the master loop so as to provide a certain offset.
Moreover, for instance, when the temperatures of three zones are stabilized in a steady state at the respective temperature setpoints (SV1, SV2, SV3)=(10° C., 20° C., 30° C.) and, subsequently, the temperature setpoints are inverted to (SV1, SV2, SV3)=(30° C., 20° C., 10° C.), the temperature in the middle zone deviates from the temperature setpoint SV2 at the moment of switching between the temperature setpoints although the temperature setpoint SV2 is supposed to be unchanged. Such an error requires more time for stabilizing the temperature to the temperature setpoint, which adversely affects throughput.